Looks like one problem with mulligans is that if a match loses a mulligan every match below it also loses a mulligan. So if you have 6 matches and you get l,w,l,w,w,w your mulligans go -1,-1,-2,-2,-2,-2 instead of -1,0,-1,0,0,0.
Even then, there's more mulligan losses around, haven't seen the exact pattern yet.
The weirdness is very visible when rocks don't lose and paper and scissor are 50-75% switched to rock. So there's only 0-1 losses per round.